ovl: lookup whiteouts outside iterate_dir()

If jffs2 can deadlock on overlayfs readdir because it takes the same lock
on ->iterate() as in ->lookup().

Fix by moving whiteout checking outside iterate_dir().  Optimized by
collecting potential whiteouts (DT_CHR) in a temporary list and if
non-empty iterating throug these and checking for a 0/0 chardev.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Fixes: 49c21e1cacd7 ("ovl: check whiteout while reading directory")
Reported-by: Roman Yeryomin <leroi.lists@gmail.com> 

1 files changed, 49 insertions, 28 deletions

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 907870e81a72..70e9af551600 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -23,6 +23,7 @@ struct ovl_cache_entry {
         u64 ino;
         struct list_head l_node;
         struct rb_node node;
+        struct ovl_cache_entry *next_maybe_whiteout;
         bool is_whiteout;
         char name[];
 };
@@ -39,7 +40,7 @@ struct ovl_readdir_data {
         struct rb_root root;
         struct list_head *list;
         struct list_head middle;
-        struct dentry *dir;
+        struct ovl_cache_entry *first_maybe_whiteout;
         int count;
         int err;
 };
@@ -79,7 +80,7 @@ static struct ovl_cache_entry *ovl_cache_entry_find(struct rb_root *root,
         return NULL;
 }
 
-static struct ovl_cache_entry *ovl_cache_entry_new(struct dentry *dir,
+static struct ovl_cache_entry *ovl_cache_entry_new(struct ovl_readdir_data *rdd,
                                                    const char *name, int len,
                                                    u64 ino, unsigned int d_type)
 {
@@ -98,29 +99,8 @@ static struct ovl_cache_entry *ovl_cache_entry_new(struct dentry *dir,
         p->is_whiteout = false;
 
         if (d_type == DT_CHR) {
-                struct dentry *dentry;
-                const struct cred *old_cred;
-                struct cred *override_cred;
-
-                override_cred = prepare_creds();
-                if (!override_cred) {
-                        kfree(p);
-                        return NULL;
-                }
-
-                /*
-                 * CAP_DAC_OVERRIDE for lookup
-                 */
-                cap_raise(override_cred->cap_effective, CAP_DAC_OVERRIDE);
-                old_cred = override_creds(override_cred);
-
-                dentry = lookup_one_len(name, dir, len);
-                if (!IS_ERR(dentry)) {
-                        p->is_whiteout = ovl_is_whiteout(dentry);
-                        dput(dentry);
-                }
-                revert_creds(old_cred);
-                put_cred(override_cred);
+                p->next_maybe_whiteout = rdd->first_maybe_whiteout;
+                rdd->first_maybe_whiteout = p;
         }
         return p;
 }
@@ -148,7 +128,7 @@ static int ovl_cache_entry_add_rb(struct ovl_readdir_data *rdd,
                         return 0;
         }
 
-        p = ovl_cache_entry_new(rdd->dir, name, len, ino, d_type);
+        p = ovl_cache_entry_new(rdd, name, len, ino, d_type);
         if (p == NULL)
                 return -ENOMEM;
 
@@ -169,7 +149,7 @@ static int ovl_fill_lower(struct ovl_readdir_data *rdd,
         if (p) {
                 list_move_tail(&p->l_node, &rdd->middle);
         } else {
-                p = ovl_cache_entry_new(rdd->dir, name, namelen, ino, d_type);
+                p = ovl_cache_entry_new(rdd, name, namelen, ino, d_type);
                 if (p == NULL)
                         rdd->err = -ENOMEM;
                 else
@@ -219,6 +199,43 @@ static int ovl_fill_merge(struct dir_context *ctx, const char *name,
                 return ovl_fill_lower(rdd, name, namelen, offset, ino, d_type);
 }
 
+static int ovl_check_whiteouts(struct dentry *dir, struct ovl_readdir_data *rdd)
+{
+        int err;
+        struct ovl_cache_entry *p;
+        struct dentry *dentry;
+        const struct cred *old_cred;
+        struct cred *override_cred;
+
+        override_cred = prepare_creds();
+        if (!override_cred)
+                return -ENOMEM;
+
+        /*
+         * CAP_DAC_OVERRIDE for lookup
+         */
+        cap_raise(override_cred->cap_effective, CAP_DAC_OVERRIDE);
+        old_cred = override_creds(override_cred);
+
+        err = mutex_lock_killable(&dir->d_inode->i_mutex);
+        if (!err) {
+                while (rdd->first_maybe_whiteout) {
+                        p = rdd->first_maybe_whiteout;
+                        rdd->first_maybe_whiteout = p->next_maybe_whiteout;
+                        dentry = lookup_one_len(p->name, dir, p->len);
+                        if (!IS_ERR(dentry)) {
+                                p->is_whiteout = ovl_is_whiteout(dentry);
+                                dput(dentry);
+                        }
+                }
+                mutex_unlock(&dir->d_inode->i_mutex);
+        }
+        revert_creds(old_cred);
+        put_cred(override_cred);
+
+        return err;
+}
+
 static inline int ovl_dir_read(struct path *realpath,
                                struct ovl_readdir_data *rdd)
 {
@@ -229,7 +246,7 @@ static inline int ovl_dir_read(struct path *realpath,
         if (IS_ERR(realfile))
                 return PTR_ERR(realfile);
 
-        rdd->dir = realpath->dentry;
+        rdd->first_maybe_whiteout = NULL;
         rdd->ctx.pos = 0;
         do {
                 rdd->count = 0;
@@ -238,6 +255,10 @@ static inline int ovl_dir_read(struct path *realpath,
                 if (err >= 0)
                         err = rdd->err;
         } while (!err && rdd->count);
+
+        if (!err && rdd->first_maybe_whiteout)
+                err = ovl_check_whiteouts(realpath->dentry, rdd);
+
         fput(realfile);
 
         return err;